It is becoming clear that many sites for water current energy systems comprise hard or rocky beds. Existing methods of drilling underwater rock sockets typically require a fixed drilling platform such as a jack-up vessel. This becomes expensive in water depths of more than approximately 30 m because it can no longer be carried out using relatively cheap and available near-shore construction jack-up vessels, but instead requires expensive specialist offshore vessels such as mobile drilling units (MDUs) or dynamically positioned (DP) drilling ships. It is possible that dedicated installation jack-ups could be developed for ocean energy installations; however, these would need to be capable of standing in water up to 70 m deep in order to capture a significant proportion of the UK tidal energy resource. This is a significant extension of current operating envelopes, and there are currently no indications that this could be achieved cost-effectively.
It is known to drill into rock using rotary drilling or percussive drilling.
The problems associated with using a conventional large diameter rotary drill are firstly that there is little if any of such equipment capable of operation underwater, and secondly that the equipment is large and relatively complex, and requires to be mounted on a structure which can react the large torques generated. The drill string is heavy and the vessel required to deploy it is correspondingly large.
Fluid-operated percussive drilling equipment is existing technology and has been used for onshore and offshore drilling. In the offshore environment, however, it has only previously been deployed using surface-breaking rigid drill stringers operated from stable drilling stringers.
There is therefore needed a cost-effective means of securing anchorages, moorings or foundations to the bed of a body of water.